Various types of gas sensors have been proposed; see for example U.S. Pat. No. 4,155,827, MAURER et al., assigned to the assignee of the present invention, in which a tubular solid electrolyte is closed at one end, the outside of the solid tubular electrolyte being exposed to the exhaust gases and the inside to ambient air, to provide a reference oxygen level. The sensor is enclosed in a tubular metal housing, which may be a two-part structure, and electrodes are applied to the inner surface as well as to the outer surface of the solid electrolyte tubular body. To provide for uniformity of output measurement, the solid electrolyte body should be operated at an essentially unvarying temperature. During start-up, and before exhaust gases, e.g. from an internal combustion engine (ICE), have reached a predetermined temperature level, a heater element is energized to raise the temperature of the solid electrolyte body to an operating temperature, for example above 250.degree. C. or 300.degree. C. Upon subsequent continued operation of the ICE, the temperature of the solid electrolyte sensing element can be maintained due to the heat of the exhaust gases.
To raise the temperature, it has previously been proposed to introduce a heater element into the interior region of the solid electrolyte body (see the referenced U.S. Pat. No. 4,155,827). The heater element may be a rod-like electrical structure. Assembling such a heater element within the interior of the solid electrolyte tubular body is complicated, and hence expensive.
Heating elements having heating coils applied to their surface by printing or similar technology and which are covered by means of thin electrical-insulating layers are already known from the referenced U.S. Pat. No. 4,035,613.